The broad definition of regicide (Latin: regis "of king" + cida "killer" or cidium "killing") is the deliberate killing of a monarch, or the person responsible for the killing of a person of royalty. In a narrower sense, in the British tradition, it refers to the judicial execution of a king after a trial, reflecting the historical precedent of the trial and execution of Charles I of England. More broadly, it can also refer to the killing of an emperor or any other reigning sovereign.

After the First English Civil War, King Charles I was a prisoner of the Parliamentarians. They tried to negotiate a compromise with him, but he stuck steadfastly to his view that he was King by Divine Right and attempted in secret to raise an army to fight against them. It became obvious to the leaders of the Parliamentarians that they could not negotiate a settlement with him and they could not trust him to refrain from raising an army against them; they reluctantly came to the conclusion that he would have to be put to death. On 13 December 1648, the House of Commons broke off negotiations with the King. Two days later, the Council of Officers of the New Model Army voted that the King be moved from the Isle of Wight, where he was prisoner, to Windsor "in order to the bringing of him speedily to justice".[2] In the middle of December, the King was moved from Windsor to London. The House of Commons of the Rump Parliament passed a Bill setting up a High Court of Justice in order to try Charles I for high treason in the name of the people of England. From a Royalist and post-restoration perspective this Bill was not lawful, since the House of Lords refused to pass it and it failed to receive Royal Assent. However, the Parliamentary leaders and the Army pressed on with the trial anyway.

At his trial in front of The High Court of Justice on Saturday 20 January 1649 in Westminster Hall, Charles asked "I would know by what power I am called hither. I would know by what authority, I mean lawful".[3] In view of the historic issues involved, both sides based themselves on surprisingly technical legal grounds. Charles did not dispute that Parliament as a whole did have some judicial powers, but he maintained that the House of Commons on its own could not try anybody, and so he refused to plead. At that time under English law if a prisoner refused to plead then this was treated as a plea of guilty (This has since been changed; a refusal to plead now is interpreted as a not-guilty plea).[4]

He was found guilty on Saturday 27 January 1649, and his death warrant was signed by 59 Commissioners. To show their agreement with the sentence of death, all of the Commissioners who were present rose to their feet.

This contemporary print depicts Charles I's decapitation.

On the day of his execution, 30 January 1649, Charles dressed in two shirts so that he would not shiver from the cold, in case it was said that he was shivering from fear. His execution was delayed by several hours so that the House of Commons could pass an emergency bill to make it an offence to proclaim a new King, and to declare the representatives of the people, the House of Commons, as the source of all just power. Charles was then escorted through the Banqueting House in the Palace of Whitehall to a scaffold where he would be beheaded.[5] He forgave those who had passed sentence on him and gave instructions to his enemies that they should learn to "know their duty to God, the King - that is, my successors - and the people".[6] He then gave a brief speech outlining his unchanged views of the relationship between the monarchy and the monarch's subjects, ending with the words "I am the martyr of the people".[7] His head was severed from his body with one blow.

One week later, the Rump, sitting in the House of Commons, passed a bill abolishing the monarchy. Ardent Royalists refused to accept it on the basis that there could never be a vacancy of the Crown. Others refused because, as the bill had not passed the House of Lords and did not have Royal Assent, it could not become an Act of Parliament.

Some regicides, such as Richard Ingoldsby were pardoned, while a further nineteen served life imprisonment. The bodies of the regicides Cromwell, Bradshaw and Ireton which had been buried in Westminster Abbey were disinterred and hanged, drawn and quartered in posthumous executions. In 1662, three more regicides John Okey, John Barkstead and Miles Corbet were also hanged, drawn and quartered. The officers of the court that tried Charles I, those who prosecuted him and those who signed his death warrant, have been known ever since the restoration as regicides.

The Parliamentary Archives in the Palace of Westminster, London, holds the original death warrant of Charles I.

Under the definition of a regicide in common usage in England, there have been two other such events since 1649: the execution of Louis XVI of France in 1793, after sentence of death by the National Convention and Maximilian I of Mexico in 1867 by a Mexican court-martial. Since Pope Sixtus V gave his broader definition of regicide and excluding monarchs killed in battle, other regicides include:

1943 Boris III, king of Bulgaria, apparent heart failure after a meeting with Adolf Hitler. According to Soviet propaganda he was poisoned by Hitler. Most probably he was poisoned after orders by Stalin to destabilize Bulgaria.

1946 Ananda Mahidol of Thailand. The King's death is still a mystery and may have been either regicide or suicide. The subject is never openly discussed in Thailand.

Regicide has particular resonance within the concept of the divine right of kings, whereby monarchs were presumed by decision of God to have a divinely anointed authority to rule. As such, an attack on a king by one of his own subjects was taken to amount to a direct challenge to the monarch, to his divine right to rule, and thus to God's will.

In the bible, David refuses to harm King Saul, because he is the Lord's anointed, even when Saul is seeking his life. And when Saul eventually is killed in battle and a person comes to David to tell him that he helped kill Saul, David puts the man to death, even though Saul had been his enemy, because he had raised his hands against the Lord's anointed. Christian concepts of the inviolability of the person of the monarch have great influence from this story. Diarmait mac Cerbaill, King of Tara (mentioned above), was killed by Áed Dub mac Suibni in 565. According to Adomnan of Iona's Life of St Columba, Áed Dub mac Suibni received God's punishment for this crime by being impaled by a treacherous spear many years later and then falling from his ship into a lake and drowning.[10]

Even after the disappearance of the divine right of kings and the appearance of constitutional monarchies, the term continued and continues to be used to describe the murder of a king.

In France, the judicial penalty for regicides (i.e. those who had murdered, or attempted to murder, the king) was especially hard, even in regard to the harsh judicial practices of pre-revolutionary France. As with many criminals, the regicide was tortured so as to make him tell the names of his accomplices. However, the method of execution itself was a form of torture. Here is a description of the death of Robert-François Damiens, who attempted to kill Louis XV:

He was first tortured with red-hot pincers; his hand, holding the knife used in the attempted murder, was burnt using sulphur; molten wax, lead, and boiling oil were poured into his wounds. Horses were then harnessed to his arms and legs for his dismemberment. Damiens' joints would not break; after some hours, representatives of the Parlement ordered the executioner and his aides to cut Damiens' joints. Damiens was then dismembered, to the applause of the crowd. His trunk, apparently still living, was then burnt at the stake.

In both the François Ravaillac and the Damiens cases, court papers refer to the offenders as a patricide, rather than as regicide, which lets one deduce that, through divine right, the king was also regarded as "Father of the country".